Condensation and dissociation rates for gas phase metal clusters from molecular dynamics trajectory calculations

Latent-to-sensible heat conversion kinetics during nanoparticle coalescence

Coagulational growth in an aerosol is a multistep process; first particles collide, and then they coalesce with one another. Coalescence kinetics have been investigated in numerous prior studies, largely through atomistic simulations of nanoclusters (102-104 atoms). However, with a few exceptions, they have either assumed the process is completely isothermal or is a constant energy process. During coalescence, there is the formation of new bonds, decreasing potential energy, and correspondingly increasing internal kinetic (thermal) energy. Internal kinetic energy evolution is dependent not only on coalescence kinetics but also on heat transfer to the surrounding gas. Here, we develop and test a model of internal kinetic energy evolution in collisionally formed nanoclusters in the presence of a background gas. We find that internal kinetic energy dynamics hinge upon a power law relationship describing latent-to-sensible heat release as well as a modified thermal accommodation coefficient. The model is tested against atomistic models of 1.5-3.0 nm embedded-atom gold nanocluster sintering in argon and helium environments. The model results are in excellent agreement with the simulation results for all tested conditions. Results show that nanocluster effective temperatures can increase by hundreds of Kelvin due to coalescence, but that the rise and re-equilibration of the internal kinetic energy is strongly dependent on the background gas environment. Interestingly, internal kinetic energy change kinetics are also found to be distinct from surface area change kinetics, suggesting that modeling coalescence heat release solely due to surface area change is inaccurate.

A neural network parametrized coagulation rate model for <3 nm titanium dioxide nanoclusters

Coagulation is a key factor governing the size distribution of nanoclusters during the high temperature synthesis of metal oxide nanomaterials. Population balance models are strongly influenced by the coagulation rate coefficient utilized. Although simplified coagulation models are often invoked, the coagulation process, particularly for nanoscale particles, is complex, affected by the coagulating nanocluster sizes, the surrounding temperature, and potential interactions. Toward developing improved models of nanocluster and nanoparticle growth, we have developed a neural network (NN) model to describe titanium dioxide (TiO₂) nanocluster coagulation rate coefficients, trained with molecular dynamics (MD) trajectory calculations. Specifically, we first calculated TiO₂ nanocluster coagulation probabilities via MD trajectory calculations varying the nanocluster diameters from 0.6 to 3.0 nm, initial relative velocity from 20 to 700 m s^-1, and impact parameter from 0.0 to 8.0 nm. Calculations consider dipole-dipole interactions, dispersion interactions, and short-range repulsive interactions. We trained a NN model to predict whether a given set of nanocluster diameters, impact parameter, and initial velocity would lead to the outcome of coagulation. The accuracy between the predicted outcomes from the NN model and the MD trajectory calculation results is >95%. We subsequently utilized both the NN model and MD trajectory calculations to examine coagulation rate coefficients at 300 and 1000 K. The NN model predictions are largely within the range 0.65-1.54 of MD predictions, and importantly NN predictions capture the local minimum coagulation rate coefficients observed in MD trajectory calculations. The NN model can be directly implemented in population balances of TiO₂ formation.

Induced Fit and Mobility of Cycloalkanes within Nanometer-Sized Confinements at 5 K

Host-guest architectures provide ideal systems for investigating site-specific physical and chemical effects. Condensation events in nanometer-sized confinements are particularly interesting for the investigation of intermolecular and molecule-surface interactions. They may be accompanied by conformational adjustments representing induced fit packing patterns. Here, we report that the symmetry of small clusters formed upon condensation, their registry with the substrate, their lateral packing, and their adsorption height are characteristically modified by the packing of cycloalkanes in confinements. While cyclopentane and cycloheptane display cooperativity upon filling of the hosting pores, cyclooctane and to a lesser degree cyclohexane diffusively redistribute to more favored adsorption sites. The dynamic behavior of cyclooctane is surprising at 5 K given the cycloalkane melting point of >0 °C. The site-specific modification of the interaction and behavior of adsorbates in confinements plays a crucial role in many applications of three-dimensional porous materials as gas storage agents or catalysts/biocatalysts.

Study on liquid-like SiGe cluster growth during co-condensation from supersaturated vapor mixtures by molecular dynamics simulation

Based on the co-condensation processes in the Si-Ge system upon cooling, as determined by molecular dynamics (MD) simulation, we explored the mixed cluster growth dynamics and structural properties leading to the synthesis of liquid-like SiGe nanoclusters. The results indicated that the cluster size quickly increased to large clusters by the coalescence of transient small clusters in the growth stage during co-condensation. The transient clusters at different temperatures were verified to have slightly Si-rich compositions and liquid-like structures. The coalescence of such nanoclusters at high temperatures led to spherical clusters with homogeneous intermixing. However, irregularly shaped clusters with attached mixed parts were obtained owing to incomplete coalescence at low temperatures. Ge atoms tended to move to the cluster surface to exploit their energetically favorable state during the restructuring process, leading to slightly Ge-rich components on the cluster surface. The degree of intermixing for SiGe nanoclusters was related to cluster size. Generally, small clusters appeared to be more segregated during restructuring.

The role of gold atom concentration in the formation of Cu-Au nanoparticles from the gas phase

The synthesis of bimetallic nanoparticles need to be controlled in order to obtain particles of a desired size, spatial structure, and chemical composition. In the synthesis of the Cu-Au nanoparticles studied here, nanoparticles can be obtained through either chemical or physical methods, each of which has its own drawbacks. Although it is very difficult to achieve the required target chemical composition of nanoparticles during chemical synthesis, their size can be stabilized quite well. In turn, physical synthesis methods mainly allow to maintain the required chemical composition; however, the size of the resulting particles varies significantly. To solve this issue, we studied the formation of Cu-Au nanoparticles with different chemical compositions from a gaseous medium using computer molecular dynamics (MD) simulation. The aim was to determine the effect of the concentration of gold atoms on the size and on the actual chemical composition of the formed bimetallic nanoparticles. The modeled region had a cubic shape with a face length of 1350 Bohr radii and contained a total of 91125 copper and gold atoms uniformly distributed in space. Thus, based on the results of the MD simulation, it was concluded that an increase in the percentage of gold atoms in the initial vapor phase led to a decrease in the size of the synthesized nanoparticles. In addition, it was found that clusters with a size of more than 400-500 atoms, regardless of the chemical composition of the initial vapor phase, basically corresponded to a given target composition.

Thermodynamics and the structure of clusters in the dense Au vapor from molecular dynamics simulation

Clusters of atoms in dense gold vapor are studied via atomistic simulation with the classical molecular dynamics method. For this purpose, we develop a new embedded atom model potential applicable to the lightest gold clusters and to the bulk gold. Simulation provides the equilibrium vapor phases at several subcritical temperatures, in which the clusters comprising up to 26 atoms are detected and analyzed. The cluster size distributions are found to match both the two-parameter model and the classical nucleation theory with the Tolman correction. For the gold liquid-vapor interface, the ratio of the Tolman length to the radius of a molecular cell in the liquid amounts to ∼0.16, almost exactly the value at which both models are identical. It is demonstrated that the lightest clusters have the chain-like structure, which is close to the freely jointed chain. Thus, the smallest clusters can be treated as the quasi-fractals with the fractal dimensionality close to two. Our analysis indicates that the cluster structural transition from the solid-like to chain-like geometry occurs in a wide temperature range around 2500 K.

Excess thermal energy and latent heat in nanocluster collisional growth

Nanoclusters can form and grow by nanocluster-monomer collisions (condensation) and nanocluster-nanocluster collisions (coagulation). During growth, product nanoclusters have elevated thermal energies due to potential and thermal energy exchange following a collision. Even though nanocluster collisional heating may be significant and strongly size dependent, no prior theory describes this phenomenon for collisions of finite-size clusters. We derive a model to describe the excess thermal energy of collisional growth, defined as the kinetic energy increase in the product cluster, and latent heat of collisional growth, defined as the heat released to the background upon thermalization of the nonequilibrium cluster. Both quantities are composed of a temperature-independent term related to potential energy minimum differences and a size- and temperature-dependent term, which hinges upon heat capacity and energy partitioning. Example calculations using gold nanoclusters demonstrate that collisional heating can be important and strongly size dependent, particularly for reactive collisions involving nanoclusters composed of 14-20 atoms. Excessive latent heat release may have considerable implications in cluster formation and growth.

Formation and fragmentation of the tungsten clusters in gas phase

We present a theoretical study of accumulation of clusters consisting of up to 100 tungsten atoms based on information extracted from molecular dynamics trajectory simulations. The description is based on the rates corresponding to the single W atom attachment to W_n clusters and their dissociation processes. The results display a strong Arrhenius dependence of the dissociation rate constant on temperature. The preferred products of dissociation of the clusters composed of more than ten atoms are single W atoms and fragments with six to nine atoms. On the other hand, the association rate constants depend weakly on temperature. The obtained rate constants are used to calculate the chemical equilibrium of the W clusters that results in significant traces of small clusters only at high initial W atoms concentrations.

Atomistic insights into the dynamics of binary collisions between gaseous molecules and polycyclic aromatic hydrocarbon dimers

The scattering mechanism of gaseous molecules on PAH dimers and their stability after collisions are investigated for the first time.